This invention relates to such electroacoustic transducers as loudspeakers or microphones and, in particular, to an electroacoustic transducer featured in an edge of diaphragm.
Generally, the loudspeaker comprises the diaphragm which is supported at its periphery through the edge, and various, optimum characteristics of the shape, material and so on have been required to the edge for attaining desired acoustic characteristics.
There have been various types of loudspeakers have been suggested, examples of which would be ones disclosed in U.S. Pat. No. 5,371,805 to Saiki et al. and Japanese Laid-Open Patent Publication No. 11-168793 of Koreeda and assigned to the same assignee as that of the present invention, in the latter of which, as shown in FIG. 7 of herein accompanying drawings, a loudspeaker comprises a pan-shaped yoke 1 having at outer periphery an upright part 2, a permanent magnet 3 disposed on inner bottom face of the yoke 1, a pole piece 4 placed on the magnet 3, voice coils 5 wound on a bobbin 6 and held in a magnetic gap between the upright part 2 of the yoke 1 and the pole piece 4, a dome shaped diaphragm 7 secured to top end of the bobbin 6, an up-roll type edge 8 secured to outer periphery of the diaphragm 7, and a frame 9 secured to outer periphery of the yoke""s upright part 2 and supporting outer periphery of the edge 8.
In the foregoing loudspeaker, the diaphragm 7 supported at the outer periphery through the edge 8 for the vibration shows an asymmetric linearity at the supporting part with respect to inputs, getting remarkable as the amplitude increases, so as to be a cause of generation of higher harmonics distortion. In the case of the up-rolled edge, for example, air volumes A1 and A2 discharged by the edge 8 upon vibration will be mutually different even if displacing amount in forward and rearward or upward and downward vibration of the diaphragm 7 is the same, as shown in accompanying FIG. 8. Further, as the displacing amount of the diaphragm 7 increases, the difference in the discharged air amount becomes larger and, in elastic control zone, the sound pressure of the loudspeaker is proportional to the sum of discharged air volumes by the diaphragm 7 and the edge 8. Due to this, there arises a problem that a difference in the sound pressure of the forward and rearward movements of the vibration increases as the vibratory amplitude of the diaphragm 7 increases and, consequently, the higher harmonics distortion increases.
A solution for the above problem has been disclosed in Japanese Patent No. 2,568,786, in which a loudspeaker 20 has an edge 21, as shown in accompanying FIGS. 9 and 10, which is divided into a plurality of parts in lengthwise, circumferential direction, such that adjacent ones 22 and 23 of these divided parts are alternately up-rolled and down-rolled to be symmetrical in section on forward and rearward sides of the circumferential direction, circumferentially spaced and oppositely rolled rims of the adjacent divided parts 22 and 23 are mutually connected with a connecting part 24 of a section sequentially, gradually varying from up-rolled shape to down-rolled shape, and the edge 21 is coupled along the inner periphery to a diaphragm 25 and along the outer periphery to a frame 26.
With such symmetrical structure as referred to in the above of the adjacent ones of the divided parts 22 and 23 in the edge 21, a composite volume of air discharged by the up-rolled divided parts and by the down-rolled divided parts upon vibration of the diaphragm 25 will be equal throughout up and down amplitudes with respect to neutral point of no input signal, at every divided parts 22 and 23 connected to any one of the connecting parts 24.
However, the diaphragm 25 driven through, as shown in FIG. 11, voice coils 28 disposed for vibratory motion in a magnetic gap 27 of magnetic circuit 26 involves a problem that magnetic flux distribution in axial direction of the gap 27 will not be uniform.
That is, the magnetic circuit shown in FIG. 11 is of a general outer magnet type comprising a yoke 29 having a center core, a ferrite magnet 30 enclosing outer periphery of the center core, and a pole piece plate 31 disposed on the magnet 30 to form the magnetic gap 27, wherein, when a center line 32 denoted by a broken line in axial, thickness direction of the gap 27 is made a reference line, the magnetic flux distribution denoted by a curve 33 in FIG. 12 shows a remarkable attenuation on upper side of the reference line 32 and a less attenuation on lower side of the line 32, so that a driving force F of the voice coils 28 moved upward will be smaller than a driving force Fxe2x80x2 of the voice coils 28 moved downward, so as to be unbalanced. While the description is made here with respect to non-linearity of the outer magnet type magnetic circuit 26, the situation applies similarly to the case of an inner magnet type.
Even when the edge 21 is made symmetrical in structure of the adjacent ones of the divided up-rolled and down-rolled parts 22 and 23, the magnetic circuit 26 shows the non-linearity, and the problem that an excellent tone quality cannot be obtained theoretically still remains unsolved.
The present invention has been suggested to solve the foregoing problems, and it is an object to provide an electroacoustic transducer having a diaphragm capable of restraining the unbalance in the vibratory driving force due to magnetic flux distribution by means of a unique structure of the edge, so as to reduce mainly the even higher harmonics distortion and to enable an excellent tone quality to be obtainable. Further, it is another object of the invention to provide an electroacoustic transducer capable of attaining a proper rigidity of configuration as compared with the foregoing prior art, and of realizing the dimensional minimization as well as thickness minimization.
The present invention attains the above object by providing an electroacoustic transducer including a diaphragm having an edge which comprises a plurality of divided portions formed at regular intervals in circumferential direction and alternately up-rolled and down-rolled with respect to a vibration plane of the diaphragm, and a plurality of sloped-plane portions respectively for connecting between mutually opposing rims of adjacent ones of the up-rolled and down-rolled divided portions, the sloped-plane portions respectively extending along each of first tangential lines drawn at regular intervals with respect to an inner peripheral line of the edge and transversing the circumferential direction at an angle a in a range defined between second tangential lines drawn from the inner periphery to bulged faces of the respective divided portions and the vibration plane, wherein the divided up-rolled and down-rolled portions being of mutually different curved-shape to provide different stiffness upon up and down movements for cancelling a difference in upward and downward driving force generated in magnetic circuit of the transducer.
Other objects and advantages of the present invention shall be made clear in following description detailed with reference to accompanying drawings.